In general, calcium carbonates are mainly classified into “natural calcium carbonates” prepared by mechanically grinding and classifying natural limestone, weathered shell or the like and “synthetic calcium carbonates” (precipitated calcium carbonates) prepared by chemically reacting limestone. Known processes for synthesizing the synthetic calcium carbonates include the carbonation process, the lime-soda process, and the soda process, among which the lime-soda process and the soda process are employed for some special applications while the carbonation process is typically employed for industrial synthesis of calcium carbonates.
Synthesis of a calcium carbonate by the carbonation process takes place by reacting quick lime and carbonic acid gas, and typically comprises a slaking step in which water is added to quick lime CaO to give slaked lime Ca (OH)2, and a carbonation step in which carbonic acid gas CO2 is injected into the slaked lime to give the calcium carbonate CaCO3. Currently, various techniques for controlling the particle shape or particle size or the like of the product calcium carbonate have been proposed by regulating reaction conditions in synthesis steps of the calcium carbonate, especially the carbonation step.
Various techniques for depositing calcium carbonate on fibers such as pulps have also been proposed. PTL 1 describes a complex comprising a crystalline calcium carbonate mechanically bonded on a fiber. PTL 2 describes a technique for preparing a complex of a pulp and a calcium carbonate by precipitating the calcium carbonate in a suspension of the pulp by the carbonation process. PTL 3 describes a technique for improving the brightness and purity of a waste paper fiber by adding a large amount of a filler for papers and paperboards to the fiber, which comprises sending a slurry of a waste paper pulp to a gas-liquid contactor where the pulp is broken down by contact with a slurry of an alkali salt travelling in a counter-flow direction to the flow direction of the pulp in a contact/breaking zone and sending a suitable reactive gas and mixing it with the precipitating filler to deposit the filler on the surface of the fiber.
In addition, PTLs 4 and 5 disclose techniques for preparing a fiber web (wet paper) in which a calcium carbonate has been efficiently incorporated by depositing the calcium carbonate in the step of forming the fiber web.
On the other hand, calcium phosphates are known as inorganic salts of calcium. Calcium phosphates are salts composed of calcium ions and phosphate ions or diphosphate ions, and about 70% of bone consists of one type of calcium phosphates known as hydroxyapatite. Calcium phosphates are used for the preparation of fertilizers and also used as food additives for the preparation of foods such as cheese. Further, calcium phosphates are also widely used as toothpastes and supplements for promoting tooth remineralization. Such calcium phosphates have high adsorbent function so that they would be usable as functional base materials such as filters if they could be complexed with a fiber. PTL 6 discloses a technique for forming a complex with a calcium phosphate and a fiber, comprising converting a cellulose fiber into a phosphate ester under high temperature conditions of 100° C. or more in advance, and immersing it in a solution containing calcium ions or the like for about 10 days, and further in a solution containing phosphate ions or the like for about 10 days.
Additionally, titanium is widely used as a photocatalytic material. However, it had drawbacks such as low retention when it is internally added to paper; decomposition of pulp fibers by the catalytic effect of titanium after long exposure to light; etc. In order to solve these problems, PTL 7 discloses that a calcium carbonate and titanium are aggregated with a high-molecular polymer in advance and used as a filler. Further, PTL 8 discloses a method for synthesizing a complex of a calcium carbonate and titanium, comprising adding titanium during the synthesis of the calcium carbonate.